Light measuring device



Feb, 2, 1937.

W. VAN B. ROBERTS LIGHT MEASURING DEVICE Filed Aug. 31, 1935 Eye INVENTOR WALTER VAN B. ROBERTS ATTORNEY Patented Feb. 2, 1937 2,069,505

UNITED STATES PATENT OFFICE LIGHT MEASURING DEVICE Walter van B. Roberts, Princeton, N. .L, assignor to Radio Corporation of America, a corporation of Delaware Application August 31, 1933, Serial No. 687,562

6 Claims. (Cl. 25027-.5)

This invention relates to light measuring detrated position and it tends to remain in this vices such as are useful as photometers in asposition after opening of the switch. The rate tronomical work or as exposure meters in the at which it returns to its normal position detaking of still or motion pictures. It has for its pends on how' fast its positive charge is removed principal object the provision of an improved and this rate of removal is, in turn, dependent photometeric device and method of operation on the electrons released from the photoelectric which make it possible to summate and indicate cathode by the light received in the tube. It is or measure with a high degree of accuracy an therefore apparent that the time taken for the average illumination not measurable by ordinary leaf M to assume its normal or discharged posi- 10 instruments without amplification. tion is a direct measure of the intensity of the 10 Another object is the provision of an improved illumination at the window of the photoelectric photometric device which is capable of operating tube. For photographic purposes, the photoelecwithout the distortional effects introduced by tric cathode i5 is exposed only to light coming vacuum tube amplifiers and the like. from the object to be photographed, and, in some In the various illustrated modifications of the instances, onlyto' light coming from a particular 15 invention, the cumulative effect of the illuminaarea. of the object. tion is transformed into electricity by means of a Fig; 2 shows the electroscope leaf I4 as susphctoelectric device, and is measured in this state pended from theungrounded anode l6 of the by an indicating element such as an electrometer, photoelectric device thus giving a combined a glow discharge tube or a highly sensitive vacphotoelectric and electrometer tube. When the uum tube meter. These indicating elements may switch I3 is closed, the rate of charging depends be utilized in connection with a chronometer or upon the illumination, the capacities of the elecmay be provided with an indicating scale. In trometer and condenser and leakage through the certain modifications, a combined photoelectric surrounding atmosphere. The tendency of leak- 3; tube and electrometer are provided for the purage is to slow down the effect on the electrometer pose of minimizing or obviating electric leakage while the tendency of thecondenser is to increase through the surrounding atmosphere. The varithis eiiect. The indicated illumination will ous illustrated forms of the invention each opertherefore tend to be more than the true value.

ate to indicate or measure an illumination which Fig; 3' shows a variable condenser l8 connected :3 has heretofore required amplification for its in parallel with the electrometer capacity. When proper determination. the switch i9 is closed and opened, a charge The invention will be better understood from proportional to the sum of the capacities of the the following description when considered in conelectrometer and condenser" is retained by the nection with the accompanying drawing and its system: Hence a longer time will be required for :77 scope will be pointed out in the appended claims. a given illumination to produce a given decrease Referring to the drawing: in the electrometer deflection. By noting the Figs. 1 to 4 illustrate various forms of the incapacity of the condenser required to decrease vention wherein a photoelectric tube is combined the deflection a given amount in a given time, with an electroscope or electrometer for indicatthe condenser dial may be calibrated to read the 4) ing or measuring the light received by the tube, proper exposure for a given type of film and Figs. 5, 6, and 7 illustrate photometric devices diaphragm. The device of Fig. 3 tends to indiwherein a glow discharge tube is arranged to cate less than the true illumination while the cooperate with a photoelectric device for producdevice of Fig. 2 tends to indicate more than the ing electrical effects dependent on the illuminatrue illumination and the mean of the two values tion, and will be very nearly correct. 45

Figs. 8 to 10 illustrate forms of devices wherein The device of Fig. 4 differs from those previa vacuum tube measuring instrument is utilized ously described in that the electrometer which to indicate or measure the illumination received forms the anode of the photoelectric device may by a photoelectric device. be rendered altogether free of leakage. In this .50 The device of Fig. 1 includes a photoelectric device, the electrometer 20 is supported in the tube in and an electroscope H which are arevacuated tube by means including an insulator ranged to be connected to a battery I2 by a 22 which may be a sulphur rod. Also mounted switch 13. A positive electric charge is imwithin the evacuated tube is a switch 23 biased parted to the electroscope by closure of the switch to its open position and arranged to be closed by and the leaf I4 is caused to assume the illusan electromagnet 24 for imparting a charge to producing a visible indication which may be readily observed and interpolated in terms of illumination units.

In order to reduce the weight of the battery required and render the apparatus more portable, a double anode photoelectric device 28 (see Fig. 6) may be provided, power being supplied to 7 this device from a battery 29 through an interrupter 30 of any suitable type, a switch 3| and transformer 32. When the switch 31 is closed, the device 28 acts as a full wave rectifier and charges the condenser 26 up to the breakdown voltage of the glow tube 21 in a time depending on the illumination of the device 28.

It is desirable that the tube 21 have a low breakdown voltage and be made very low so that a small capacity C will be sufficient to make the discharge visible. For measurement, the incoming light may be reduced by a calibrated diaphragm to produce a flash per second, for example. The setting of the diaphragm then indicates the light value and for lower illumination, the seconds per flash may be counted.

The apparatus of Fig. '7 is similar to that of Fig. 6 with respect to the parallel circuit including the condenser 26 and the glow tube 21 but difiers therefrom in that the comparatively large low voltage current of an electrolytic. device 33 is supplied to the parallel circuit through an interrupter and a rectifier 35, a choke coil 36 being connected across the circuit intensifying the rectifier voltage.

In the devices of Figs. 8 to 10, a vacuum tube measuring instrument including a tube 31 and an indicator 38 is provided. The arrangement of Fig. 8 includes a two-Way switch 39 which is closed to the left to charge the condenser i8 and to the right to apply voltage to the output circuit of the tube 37. Since the condenser is connected to the input circuit of the tube and its charge is dependent on the illumination of the device l0, it will be apparent that the output current of the tube 31 is a measure of the illumination. The same is true of the device of Fig. 9 which includes a switch IQ for applying a charge to the condenser 26 and that of Fig. which includes a switch 40 for connecting a plurality of condensers 26 in parallel with each other and with a resistor 4! to an electrolytic device 33 and for connecting these condensers in series with one another to the input circuit of the tube 3?. It will be understood that a resistor 42 may be connected in the tube input circuit when the condensers are disconnected therefrom and that other changes may be made without surrendering the advantages made possible by the invention.

Having thus described my invention, I claim:

1. A light responsive device comprising an electron emissive cathode including within an evacuated container, an anode insulated from said cathode and including means for measuring an electric charge, means mounted within said container for transmitting at will an electric charge to said measuring means, and means external to said container for controlling the operation of said charge connecting means.

2. A photometer including a light responsive device of the electron emitting type provided with an evacuated container, electric charge measuring means mounted within said container, and means mounted within said container for transmitting an electric charge to said measuring means.

3. The combination of a light responsive device including an evacuated container, a photoelectric cathode, an anode insulated from said cathode and including means operable to indi cate the magnitude of an electric charge, and means mounted within said container for transmitting a positive charge to said anode.

4. The combination of a light responsive device of the electron emitting type including an evacuated container, a photoelectric cathode, an anode including means for measuring an electric charge and insulated from said cathode, and movable means mounted within said container for transmitting an electric charge to said anode.

5. A light responsive device comprising an evacuated container, an anode therein including means for measuring an electric charge, and a photoelectric cathode in said container.

6. A photo tube comprising a photo-electric cathode, an anode comprising an electroscopc. and an evacuated container enclosing both of said elements.

WALTER VAN B. ROBERTS. 

